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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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THE EARTHQUAKE ADVANCE THEORY         183
vance of these glaciers, and in formulating a theory of the nature of the advance we naturally start with the results of their work as a basis. The advance in the two regions is closely analogous in main characteristics though differing in cause, rate of transformation, and amount of forward motion; but the two latter differences may well be no more than what would normally result from the difference in cause,—in the one case the slow accumulation of energy and ultimate response from increase in snowfall, in the other, rapid application of the same cause due to sudden precipitation of snow into the reservoirs from the valley walls and corresponding rapid and great response in the lower glacier.
In explaining such an advance of glaciers as those described in this and preceding chapters we cannot assume actual flowage or transfer of ice from reservoir to terminus because of the brevity of the time and the abruptness with which the advance passed through the glaciers. It is inconceivable, for instance, that there was a transfer of material through the 15 miles of the broken Marvine Glacier, in the brief period of a year or less. Snow that falls in glacier reservoirs, changed to ice, does by normal glacier motion ultimately, in the course of a score of years, find its way to the glacier front, but we imagine that no one would advance the hypothesis that glacier motion could under any circumstances become so accelerated as to permit such transfer as would be called for by the Alaskan glacier advances.
A modification of this hypothesis that is less difficult to conceive is that, by increase of snow in the reservoir, a condition of advance is started in the upper glacier, and that this is so great and powerful that it results in a rapid actual forward transfer of ice, causing motion in the glacier of far greater rapidity than normal. By this rapid advance the glacier surface is broken and a thrust is applied against the ice in front which is not yet in such rapid motion. This thrust pushes the ice forward and thus extends the breaking and advance beyond the zone actually reached by the ice which ia moving rapidly forward. In other words, this hypothesis takes as the basis the normal means of transfer of ice in glaciers, but supposes an acceleration in speed, and adds to it the pushing forward and consequent breaking up of the rigid stagnant outer ice, much as an advancing glacier would push forward the wall of a building that stood in the way.
In 1906 this hypothesis seemed, in part at least, a possible explanation of the conditions in the advancing glaciers of Yakutat Bay. In the light of the observations of 1909, and after further consideration of the problem, it now seems to us an inadequate explanation, for there are several serious objections to it. In the first place, it is doubtful if, even with the most rapid glacier motion we could assume, the wave of advance could have extended as far down the glacier by 1906 as this explanation would demand; and, in any event, it should have been preceded by breaking in the upper glacier, and not have abruptly affected the entire glacier, for many miles, all in a period of a few months. Furthermore, it seems exceedingly doubtful whether sufficient thrust could be applied to so completely break the surface of such extensive areas as were broken, for instance in the Marvine lobe of the Malaspina by the 1906 advance. Even more difficult to conceive is the application of a thrust of sufficient force to push the front of Hidden Glacier forward two miles. Finally, the fact that the advance ceased abruptly and completely immediately after the breaking of the ice was accomplished is a serious objection. Had there been an active advance from behind, pushing